1. Field of the Invention
The present invention relates to a machine tool for rotating a workpiece about a predetermined axis, and moving at least one of the workpiece and a tool for machining the workpiece, thereby machining the workpiece into a desired shape, and a control method therefor.
2. Related Background Art
As a machine tool of this type, machine tools equipped with numerical controllers have conventionally been known, and include an arrangement suitable for high-speed cutting and an arrangement suitable for high-accuracy machining. A machine tool having an arrangement suitable for high-speed cutting rotates a workpiece at a higher speed than that in high-accuracy machining, and increases the relative moving speed of a tool to machine the workpiece. For this reason, the rotational speed of the workpiece readily varies due to an increase and variations in cutting resistance generated in machining. The machining accuracy of the workpiece tends to decrease owing to deflection of the tool or the like.
On the other hand, a machine tool having an arrangement suitable for high-accuracy machining decreases the rotational speed of a workpiece in contrast to high-speed cutting to suppress the influence of the cutting resistance generated in machining, and synchronizes the rotational speed of the workpiece with the relative moving speed of the tool to strictly position the tool with respect to the workpiece. In the machine tool having an arrangement suitable for high-accuracy machining, an increase in machining efficiency such as the machining speed is limited, and the machining efficiency depends on the performance of a numerical controller.
For example, when a workpiece is to be threaded, the thread must be shaped to advance the screw by one lead by one rotation of the screw, and thread cutting requires high machining accuracy. Generally in a thread cutting machine tool, the rotational speed of a workpiece is set as low as about 1,500 rpm so as to thread the workpiece at high machining accuracy. Note that the screw has a tolerance class such as the first, second, or third class that is determined based on the machining accuracy such as the dimensional tolerance or tolerance.
Some of screw components including a wood screw do not require high machining accuracy. For example, when wood screws are to be mass-produced, the thread cutting machine tool has an arrangement suitable for high-accuracy machining, as described above, so the machining speed cannot be increased, resulting in low productivity. To increase the productivity, a wood screw may be machined using a machine tool having an arrangement suitable for high-speed cutting. However, the machine tool having an arrangement suitable for high-speed cutting cannot machine screws other than a wood screw that require high machining accuracy.
The present invention has been made in consideration of the above situation, and has as its object to provide a machine tool capable of performing high-speed machining and high-accuracy machining and increasing the productivity, and a control method therefor.
According to the present invention, a machine tool for rotating a workpiece about a predetermined axis, and moving at least one of the workpiece and a tool for machining the workpiece, thereby machining the workpiece into a desired shape is characterized by comprising pulse signal generation means for generating a pulse signal every predetermined rotational angle at the predetermined axis, first moving position determination means for determining a moving position of at least one of the workpiece and the tool every predetermined rotational angle, first movement control means for controlling movement of at least one of the workpiece and the tool in correspondence with the pulse signal generated by the pulse signal generation means so as to set the moving position of at least one of the workpiece and the tool to the moving position determined by the first moving position determination means, divided-timing signal generation means for generating a divided-timing signal every predetermined time interval, second moving position determination means for determining the moving position of at least one of the workpiece and the tool every predetermined time interval, second movement control means for controlling movement of at least one of the workpiece and the tool in correspondence with the divided-timing signal generated by the divided-timing signal generation means so as to set the moving position of at least one of the workpiece and the tool to the moving position determined by the second moving position determination means, and control selection means for selecting one of the first and second movement control means to control movement of at least one of the workpiece and the tool.
In the machine tool according to the present invention, when the control selection means selects control by the first movement control means, the first movement control means controls movement of at least one of the workpiece and tool in correspondence with a pulse signal generated by the pulse signal generation means so as to move at least one of the workpiece and tool to the corresponding moving position determined by the first moving position determination means. Then, the moving position of at least one of the workpiece and tool is controlled in synchronism with rotation of the workpiece about a predetermined axis. As a result, even if the rotational speed of the workpiece about the predetermined axis varies, the workpiece and tool can be strictly positioned with respect to each other, which can realize high-accuracy machining.
In the machine tool according to the present invention, when the control selection means selects control by the second movement control means, the second movement control means controls movement of at least one of the workpiece and tool in correspondence with a divided-timing signal generated by the divided-timing signal generation means so as to move at least one of the workpiece and tool to the corresponding moving position determined by the second moving position determination means. Then, the moving position of at least one of the workpiece and tool is controlled without synchronization with rotation of the workpiece about the predetermined axis. Although high-accuracy machining cannot be achieved, the workpiece can be machined at a machining accuracy corresponding to the rotational speed even when the rotational speed of the workpiece about the predetermined axis is high. In addition, when the control selection means selects control by the second movement control means, the rotational speed of the workpiece about the predetermined axis can be increased to perform high-speed machining and increase the machining efficiency.
From this, the machine tool according to the present invention enables high-speed machining and high-accuracy machining in one machine tool, thus increasing the productivity.
The machine tool according to the present invention may be characterized in that the control selection means selects one of the first and second movement control means on the basis of a predetermined machining condition of the workpiece. Movement of at least one of the workpiece and tool can be optimally controlled under the machining condition of the workpiece.
The machine tool according to the present invention may be characterized in that the predetermined machining condition of the workpiece may include thread machining accuracy in thread cutting. This enables efficient machining complying with the quality required in thread cutting.
The machine tool according to the present invention may be characterized in that the machine tool further comprises counting means for counting the number of times of generation of pulse signals generated by the pulse signal generation means, and the control selection means switches one to the other of the first and second movement control means when the number of times of generation of pulse signals counted by the counting means reaches a predetermined value. Accordingly, the movement control means can be switched even at the intermediate portion of the shape of the workpiece. As a result, even when the workpiece partially includes a portion requiring high machining accuracy, control by the first movement control means can be selected and switched to perform work at high machining accuracy. At a portion other than the portion requiring high machining accuracy, control by the second movement control means can be selected and switched to perform work at high machining efficiency. From this, even in machining a workpiece partially including a portion having a different machining accuracy, the workpiece can be efficiently machined in accordance with the required quality.
According to the present invention, a control method for a machine tool for rotating a workpiece about a predetermined axis, and moving at least one of the workpiece and a tool for machining the workpiece, thereby machining the workpiece into a desired shape, the machine tool including pulse signal generation means for generating a pulse signal every predetermined rotational angle at the predetermined axis, first moving position determination means for determining a moving position of at least one of the workpiece and the tool every predetermined rotational angle, first movement control means for controlling movement of at least one of the workpiece and the tool in correspondence with the pulse signal generated by the pulse signal generation means so as to set the moving position of at least one of the workpiece and the tool to the moving position determined by the first moving position determination means, divided-timing signal generation means for generating a divided-timing signal every predetermined time interval, second moving position determination means for determining the moving position of at least one of the workpiece and the tool every predetermined time interval, and second movement control means for controlling movement of at least one of the workpiece and the tool in correspondence with the divided-timing signal generated by the divided-timing signal generation means so as to set the moving position of at least one of the workpiece and the tool to the moving position determined by the second moving position determination means, is characterized by comprising the steps of: selecting the first movement control means in machining the workpiece at high machining accuracy, and the second movement control means in machining the workpiece at low machining accuracy; and decreasing a rotational speed of the workpiece about the predetermined axis in machining the workpiece at the high machining accuracy, and increasing the rotational speed of the workpiece about the predetermined axis in machining the workpiece at the low machining accuracy in comparison with machining the workpiece at the high machining accuracy.
In the machine tool control method according to the present invention, when the workpiece is machined at high machining accuracy, the rotational speed of the workpiece about a predetermined axis is decreased, and control by the first movement control means is selected. The first movement control means controls movement of at least one of the workpiece and tool in correspondence with a pulse signal generated by the pulse signal generation means so as to move at least one of the workpiece and tool to the corresponding moving position determined by the first moving position determination means. Then, the moving position of at least one of the workpiece and tool is controlled in synchronism with rotation of the workpiece about the predetermined axis. As a result, even if the rotational speed of the workpiece about the predetermined axis varies, the workpiece and tool can be strictly positioned with respect to each other, which can realize high-accuracy machining.
In the machine tool control method according to the present invention, when the workpiece is machined at low machining accuracy, control by the second movement control means is selected. The second movement control means controls movement of at least one of the workpiece and tool in correspondence with a divided-timing signal generated by the divided-timing signal generation means so as to move at least one of the workpiece and tool to the corresponding moving position determined by the second moving position determination means. Then, the moving position of at least one of the workpiece and tool is controlled without synchronization with rotation of the workpiece about the predetermined axis. Although high-accuracy machining cannot be achieved, the workpiece can be machined at a machining accuracy corresponding to the rotational speed. Moreover, when the workpiece is machined at low machining accuracy, the rotational speed of the workpiece about the predetermined axis is higher than that in high-speed machining. High-speed machining can therefore be done to increase the machining efficiency.
From this, the machine tool control method according to the present invention enables high-speed machining and high-accuracy machining in one machine tool, thereby increasing the productivity.
The present invention will be fully understood from the following detailed description taken in conjunction with the accompanying drawings, which are merely examples and do not limit the present invention.
The further application range of the present invention will be apparent from the following detailed description. The detailed description and specific cases will represent a preferred embodiment of the present invention, but are merely examples. Various modifications and improvements within the spirit and scope of the present invention will be apparent to those skilled in the art from the following detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.